1. Field of the Invention
This invention relates generally to angular rate sensors of the vibrating element type, and particularly to an angular rate sensor having a single integrated driving and sensing element.
2. Description of the Prior Art
U.S. Pat. No. 2,513,340 to Lyman discloses the use of flexure-sensitive Rochelle salt crystals as strain sensing elements connected to an axially rotating tuning-fork whose oscillations are magnetically induced. The Lyman '340 device measures rate of turn or change in orientation of a body to which the tuning-fork is attached, and represents an early example of the use of strain-sensitive piezoelectric type crystals in an angular gyroscope or rate sensing application.
Various types of angular rate sensors employing piezoelectric crystal elements (or transducers) that are electrically excited to induce vibration are known to the art.
The use of vibrating piezoelectric crystal elements in gyroscopes and angular rate sensing devices can be traced back at least to U.S. Pat. No. 2,716,893 to Birdsall, which discloses paired transducers mounted in diametric opposition on a rotor to provide two-axis angular rate or acceleration measurement with directional specificity obtained by phase difference calculations. The Birdsall '893 device provided the means for constructing mass-independent navigation and guidance gyroscopes, but did not result in practical systems for applications such as fixed-position north-seeking gyroscopes or navigational attitude and heading referencing until the development of suitable control and filtering circuits. Representative examples of operational embodiments for those applications are shown in U.S. Pat. Nos. 3,987,555 to Haagens and 4,444,053 to Rider.
In particular, angular rate sensors having dual drive and sensing elements disposed in a "tuning-fork" configuration are the primary subject of current development activity. Some vibrating element angular rate sensors of this type, and representative drive circuits and applications for those sensors, are disclosed in U.S. Pat. Nos. 4,479,098; 4,578,650; and 4,628,734 to Watson; U.S. Pat. No. 4,671,112 to Kimura; U.S. Pat. No. 5,038,613 to Takenaka; U.S. Pat. No. 5,014,554 to Terada, and U.S. Pat. No. 4,791,815 to Yamaguchi.
These vibrating element angular rate sensors are usually characterized by a pair of parallel drive elements attached to an intermediate bridge member, with a sensing element attached to the distal end of each drive element and oriented orthogonal to that corresponding drive element. One or both of the drive elements are electrically excited to induce flexure therein in order to energize the sensing elements, and cause them to vibrate back and forth in opposition to one another within an inertial plane at a resonant frequency.
Tuning-fork type sensors can be very labor intensive to fabricate due to the bonded and insulated joints that must be formed between the bridge member, drive elements, and sensing elements, and the need to properly orient the drive and sensing elements relative to one another. The joints and the construction of the bridge member can also lead to abnormalities and imperfections that will affect the accuracy of the angular rate sensor if not filtered or corrected electronically. These devices are also subject to damage from handling during fabrication and the high G-forces imparted on the elements due to shocks or rapid acceleration and deceleration.
Other types of angular rate sensors utilizing vibrating transducers are also known to the art. One representative example is the "cantilever" type configuration shown in U.S. Pat. No. 3,842,681 to Mumme, in which six paired transducers extend radially from a hub having a central axis of rotation. Two drive transducers are oriented with their planar faces parallel to the axis of rotation and normal to their angular velocity vector, and impart vibratory oscillations to the two remaining pairs of sensing transducers. The two pairs of sensing transducers are oriented orthogonal to one another, with their planar faces parallel to their angular velocity vectors and perpendicular to the axis of rotation.
However, the Mumme '681 device requires a very complicated suspension system that does not operate effectively, and the long warm-up time of several minutes necessary to establish predominance of the primary torsional oscillations over the lateral deflection vibrations has prevented development of a production model for angular rate sensing applications.
Composite rate sensing systems incorporating single or paired vibrating tuning-forks and vibrating cantilever structures with constrained seismic masses are also know. Representative examples of such systems are shown in U.S. Pat. Nos. 2,544,646 to Barnaby and 4,802,364 to Cage, as well as Great Britain Patent Specification No. 1,540,279 to Philpotts.
Various "vibrating beam" angular rate sensors utilizing a plurality of vibrating transducers mounted on a rigid elastic core are also known to the art. These angular rate sensors provide separate transducers for driving and sensing, with the structural and physical properties of the core defining the flexural characteristics of the angular rate sensor and the vibrational modes of the transducers.
One representative example is shown in U.S. Pat. No. 3,520,195 to Tehon, which discloses an angular velocity sensing device having a central core or body with a square cross section, and a plurality of transducers bonded or soldered to each of its longitudinal surfaces. Each transducer has a pair of spaced-apart silver electrodes bonded or soldered to the outer surface opposing the central body through which the transducer can be electrically excited, or which will develop a voltage in response to flexure of the transducer induced by rotation of the central body about its longitudinal axis. A first pair of opposing transducers are energized to vibrate the central body at a resonant frequency, while the second pair of opposing transducers act as the sensing or read-out elements. Tehon '195 particularly discloses a hard or invariable metal rod such as stainless steel for the central body, and suggests that a central body having a circular or polyhedral cross section would also be operable.
Tehon '195 discloses a mounting structure in which the ends of the central body are clamped one resonant wavelength apart (defining an acoustic node located at the midpoint of the central body), and an alternate mounting structure in which the central body is directly supported at the natural acoustic nodes (one half resonant wavelength apart and inward from each end approximately 0.224-0.226 times the length of the central body) to eliminate the reflected acoustic energy created by clamping both ends. The Vyro.RTM. inertial angular rate sensor produced by General Electric constituted a practical implementation of the nodal point mounting of the Tehon '195 device. Vibrating beam systems are typically used for measuring rotational accelerations and velocities, with a representative circuit for the dynamic system analysis of such a vibrating beam accelerometer being shown in U.S. Pat. No. 4,761,743 to Wittke.
Vibrating beam sensors of this type are subject to certain drawbacks. Since the core is a rigid elastic body, it is subject to mechanical fatigue and structural unreliability. The significant amount of tuning required by the system must be accomplished through electronic compensation. Since the same resonant driving and sensing frequencies are used to increase sensitivity, the system is very susceptible to bias error and scale factor shift caused by temperature changes. Thermal expansion of the beam will also cause the nodal points and resonant frequency of the system to shift physically. Attaching the transducers to a uniform shape beam will itself shift the location of the nodal points and affect the resonant frequency, thus requiring experimental evaluation or complex theoretical analysis to determine the true nodal points and resonant frequency to correct for discrepancies and variations as the angular rate sensor expands or contracts. Imperfections in the beam can cause twisting which will produce erroneous sensing signals. The amplitude of the output voltage from the sensing element is itself so minute that extremely high amplification is required, thereby increasing noise and temperature-related bias. Bonding the transducers to the beam can result in misalignment and structural imperfections which interfere with performance, and the bonds tend to fatigue and deteriorate at different rates along the length of each transducer or relative to other transducers on the same beam, thereby interfering with the proper transmission of drive energy from the drive elements to the beam or the complete and uniform flexure of the sensing elements. The transducers are small relative to the physical size of the core, and the drive activation area for the system is therefore correspondingly limited. Additionally, the physical presence of the vibrating core makes the system susceptible to external magnetic fields which induce eddy currents that cause core vibrations in the sensitive direction.
Recent variants of the vibrating beam type angular rate sensor utilize metal cores having uniform triangular, square, or hexagonal cross sections, or a non-uniform quadrangular prismatic cross section. In practical embodiments using a core having a triangular cross section, a pair of transducers for driving and sensing are mounted on two adjacent longitudinal surfaces with a single drive detecting transducer mounted on the remaining longitudinal surface. The core is suspended near its nodal points from inverted U-shaped metal supports. Each longitudinal edge or ridgeline of the triangular core can be trimmed to raise the resonant frequency in one direction, and the drive and sensing frequencies can therefore be matched so that resonant frequencies in the X- and Y-directions are equal. Trimming all the ridgelines results in a core having a non-uniform hexagonal cross section. Systems of this general type are discussed in Japanese Patent Application Nos. 3-150,914; 2-223,818; 2-266,215; 2-266,601; 3-13,006; 3-34,613; and 59-51,517; and have been implemented in the Gyrostar.TM. angular rate sensor by Mura Tech Manufacturing Company.
Such systems have many of the structural drawbacks found in the Tehon '195 device since the piezoelectric crystals are still bonded to a rigid elastic core in the same manner as Tehon '195. While the ability to trim the ridgelines of the core can facilitate some mechanical tuning that is otherwise accomplished through electronic compensation, this mechanical tuning must be accomplished manually for each sensor produced. These sensor systems typically have a high Q-value and narrow operational bandwidth, making them unsuitable for many applications. Matching the driving and sensing frequencies does increase the sensitivity of the system by its Q-value, but also increases the sensitivity to temperature change and particularly to resonant or on-frequency vibrations. While one could compensate for having a lower Q-value by driving the system harder, there are practical limits posed by the capabilities of the drive circuit electronics and the fatique properties of the bar. In addition, these sensors vibrate freely in the longitudinal direction parallel with the major axis of the core.
The concept of mounting a vibrating transducer on a rigid elastic core has also been extended to cores having a tuning-fork shape for use in devices such as acoustic resonators. Representative examples of such structures are shown in U.S. Pat. Nos. 4,178,526 and 4,472,654 to Nakamura.
U.S. Pat. Nos. 3,258,617 to Hart and 4,489,609 to Burdess each disclose a gyroscopic or inertial rotation measuring device having a matrix of electrodes adhered or disposed on the outer surfaces and extending around the edges of a piezoelectric beam, and operating in the shear mode with forced double resonance. Hart '617 further discloses a layered construction with a pair of electrodes extending partially into the interior of the beam from opposing sides. The particular locations and shapes of the electrodes in the matrix are complex and difficult to construct. In Hart '617, the piezoelectric device is mounted within an aperture in a collar disposed at the center of the beam to produce a center nodal point and forced vibrations or oscillations of equal magnitude on the opposing sides of the collar and ends of the beam. In Burdess '609, the piezoelectric device is fixedly mounted at opposing ends to produce two end nodal points and a center nodal point, with the forced vibrations or oscillations on each side of the center nodal point being equal in amplitude but opposite in direction. Each of these devices is subject to the drawbacks discussed above relating to the beam-type devices such as Tehon '195, and a configuration such as the Burdess '609 device is particularly susceptible to torsional vibration and noise.
Accelerometers and angular rate sensing devices employing piezoelectric crystals having either a unitary tall toroidal structure or composed of stacked disks for vibrating a seismic mass are also known. Representative examples of such devices are shown in U.S. Pat. Nos. 5,052,226 and 4,586,377 to Schmid, and U.S. Pat. Nos. 3,636,387; 3,614,487; and 3,482,121 to Hatschek.
Several types of systems for angular rate or acceleration sensing that utilize vibrating bodies energized by means other than piezoelectric crystal elements are also known to the art.
One type is the vibrating wire rate sensor, in which a thin metal wire suspended from fixed ends is vibrated at its primary resonant frequency in one plane using; a drive magnet which surrounds a portion of the wire. A signal magnet is oriented to detect vibrations in the plane perpendicular to the drive plane which are induced by Coriolis forces caused by rotation of the wire. The amplitude of those vibrations will be proportional to the rate of rotation, and the phase shift of the vibrations will indicate the direction of rotation. Representative examples of vibrating wire angular rate sensors are shown in U.S. Pat. No. 3,520,193 to Granroth and U.S. Pat. Nos. 3,515,003 and 3,504,554 to Taylor.
Because the vibrating wire is fixed at both ends, the system produces a significant amount of reflected acoustic energy, as well as transferring significant mechanical energy through the supports to the surrounding structure. At least a portion of this energy can be reflected back to the sensor in the perpendicular plane, and create an erroneous sensing signal. While the erroneous signal will cause a relatively constant bias error in a fixed acoustic environment, temperature fluctuations and acceleration of the system will induce changes in the environment and the bias error can become very unpredictable. Complex mounting systems can reduce but not eliminate these signal errors. Also, the vibrating wire system does not provide a natural nodal support when used for rate sensing, so random external vibrations can create significant noise affecting the integrity of the output signal.
Another type of system employs magnetostrictive forces to induce flexure in an isotropic elastic body, with the Villari effect producing an output signal detectable by sensing magnets or electromotive devices. U.S. Pat. Nos. 2,455,939 to Meredith; 2,817,779 to Barnaby; 2,974,530 to Jaouen; 3,177,727 to Douglas; 3,127,775 to Hansen; and 3,182,512 to Jones disclose a wide variety of angular velocity measuring devices in which resonant vibrations are induced in a magnetostrictive bodies (including bars, rods, tubes, hollow cylinders, and tuning-forks) by placing the bodies in a permanent or constant magnetic field, and then applying an alternating current to excite the bodies (or magnetically responsive elements attached to the bodies.) Flexure or vibration of the bodies within the magnetic field combined with movement of the bodies out of their inertial or vibratory plane produces variations in the magnetic flux lines within opposing parts of the body that are proportional to the angular velocity of the system. Jaouen '530 discloses several embodiments, including cylindrical rods having fixed ends mounted within a base, a tuning-fork structure, and a single cylindrical rod suspended at its nodal points. In this latter embodiment, the rod is either suspended by crossed wires passing through perpendicularly bored holes at the nodal points, or by spring steel needles welded at the nodal points, to permit two degrees of freedom perpendicular to the longitudinal axis.
Due to their cumbersome physical structures and the limitations imposed by their drive or sense signal processing circuitry, the vibrating wire and magnetostrictive systems described above are generally disfavored for modern rate sensing applications as compared to vibrating piezoceramic crystal element systems.